Cellulose fibres have since long ago been used as reinforcement in laminates, compression moulding compounds and the like in combination with resins based on formaldehyde, for example phenol-formaldehyde-, melamine-formaldehyde- and urea-formaldehyde resins. It has not been possible to combine other resins such as polyester- and epoxi resins with cellulose fibres so that a reinforcement effect has been acheived. However cellulose, wood powder etc. have been used as filling materials for making the material cheaper.
Cellulose fibres have very good inherent tensile properties. The E-module for cellulose I is about 140 GPa, which makes the material to one of the strongest. For cellulose II the theoretic module is 90 GPa. Regenerated cellulose fibres are manufactured with a module of 48 GPa, while cotton has a module of 5-11 GPa. As a comparisson it can be mentioned that E-glass fibres have an E-module of 70 GPa. With regard to the high density of glass fibres, i.e. if the specific modules are compared, cellulose fibres are well comparable with glass fibres. Besides the break elongation is higher for cellulose and the price is lower.
The main volume of composite materials are today manufactured with glass fibres as reinforcement. The glass fibre per se is not compatible (i.e. is not wetted) with polyester resins but is always treated in order to improve wetting and preferably provide chemical (covalent) bond between fibre and matrix. Such a bond gives optimal mechanical properties to the composite material. It is common that the glass fibres are treated with a vinyl silane which gives a surface with projecting vinyl groups. These can copolymerize with the polyester resin and accordingly give a convalent bond between the fibre and the matrix. A corresponding treatment of cellulose fibres is however not practiced.
In the Swedish patent No. 6809101-6 (publication No. 372.533) is described a method of manufacturing a cellulose polymer with dihalogene-s-triazinyl groups bonded thereto. At the method cellulose having free hydroxyl groups, i.e. has been reacted with an alkaline acting agent, is treated with a cyanuric halide while maintaining an acid pH. With this method large amounts of reagent is required, in the stated examples the same amount by weight of reagent as cellulose. The yield is in spite of this poor. This depends on the fact that the reagent, which is added dissolved in acetone, has reacted with the hydroxyl groups in the water solution, which thus competes with the hydroxyl groups of the cellulose. The method is therefore not economically justifiable on a large scale.